MECHANISM OF ACTION OF COENZYME B12 . HYDROGEN TRANSFER IN ISOMERIZATION OF METHYLMALONYL COENZYME-A TO SUCCINYL COENZYME-A

The fate of the hydrogen atom that migrates during the isomerization of methylmalonyl coenzyme A to succinyl coenzyme A catalyzed by methylmalonyl coenzyme A isomerase, an enzyme which requires coenzyme B32 as a cofactor, has been studied by a method which follows the statistical partitioning of deuterium between substrate and product and compares the observed distribution to distributions calculated on the basis of various mechanistic models. The method is described, and the results of its application to the methylmalonyl coenzyme A mutase reaction lead to the conclusion that the hydrogen removed from methylmalonyl coenzyme A (which we find to be the rate-determining step in the overall conversion) becomes one of three equivalent hydrogens before a hydrogen is returned to regenerate methylmalonyl coenzyme A or to yield succinyl coenzyme A. The implications of these results are discussed in terms of involvement of the bond between C-5′ of the deoxyadenosine residue and the cobalt atom of the coenzyme B12.. © 1969, American Chemical Society. All rights reserved.